1. Field of the Invention
The present invention relates to a developing method and a developing assembly.
2. Description of the Related Art
A number of methods are conventionally known as methods for electrophotography in which, in general, copies are obtained by forming an electrostatic latent image on a latent image bearing member by utilizing a photoconductive material and by various means, subsequently developing the electrostatic latent image with toner to form a toner image as a visible image, transferring the toner image to a recording medium such as paper as needed, and thereafter fixing the toner image onto the recording medium by the action of heat or pressure or the like.
Image forming apparatus using the above electrophotography include copying machines, printers and so forth. In recent years, these printers and copying machines are increasingly changed over from analogue types to digital types. Developing systems are also required to be of higher definition, and are sought to be excellent in reproducibility of latent images and high in image quality as being free of toner scatter. Accordingly, the particle diameter of toner is increasingly reduced to cope with such situations.
Toners having a small particle diameter have a large surface area per unit mass, and hence tend to have high electric charges on toner particle surfaces in the step of development. Where a toner tends to have high electric charges on toner particle surfaces, the toner applied on a developer carrying member may have too large charge quantity because of contact with the developer carrying member when the developer carrying member is repeatedly rotated. A phenomenon in which the toner comes to have an excess charge quantity is called a charge-up phenomenon.
Once this charge-up phenomenon occurs, the toner and the developer carrying member surface attract each other because of mirror force that acts therebetween. Hence, the toner comes to stand immobile on the developer carrying member surface to become difficult to move from the developer carrying member to the latent image formed on the photosensitive drum. In particular, this phenomenon tends to occur in an environment of low humidity.
The toner present on the developer carrying member surface in the immobile state makes it difficult for other toner to gain access to the developer carrying member, and as a result, the toner may come to be difficult to charge. For this reason, the toner involved in development decreases, and hence problems are raised such as thin line images, decrease in image density of solid images, sleeve ghosts and density non-uniformity. Such toner not properly charged because of the charge-up may become uncontrollable to flow out on the developer carrying member to cause what is called a blotch phenomenon in which blotchy or wavy non-uniformity comes about on images.
Printers for personal use in homes and offices or for SOHO (small office home office) are often used in a low print percentage and for printing one or a few sheets. Where the number of sheets printed at a time is few (hereinafter also called “intermittent mode”), the developer carrying member is repeatedly rotated in a larger number of times than in continuous printing on a large number of sheets, to tend to cause the above charge-up phenomenon.
As a measure to cope with the charge-up phenomenon on the side of the developer carrying member, Japanese Patent Application Laid-open No. H08-240981 discloses a method in which a developer carrying member is used which is obtained by forming on a metallic substrate a resin coat layer in which a conductive substance or solid lubricant such as carbon black or graphite and conductive spherical carbon particles have been dispersed in a resin. However, this developer carrying member may be insufficient in respect of the performance to provide toner with charges rapidly and uniformly and the ability to provide toner with charges appropriately, because the profile of surface unevenness of the resin coat layer formed on the developer carrying member surface is not sufficiently uniform.
Meanwhile, the main bodies of printers and copying machines are increasingly miniaturized. Personal printers are especially strongly desired to be miniaturized, and not only their main bodies but also their developing assemblies themselves are required to be made compact. With such a trend, their component parts including developer carrying members are also increasingly made small-sized. However, taking notice of a developer carrying member used when a magnetic toner is used, making the developer carrying member smaller is to make the diameter of the developer carrying member smaller, which means that a magnet roller set in the developer carrying member is also made smaller. In this case, with a decrease in diameter of the magnet roller, the magnetic flux density decreases necessarily, and this tends to cause fog greatly in a low-temperature and low-humidity environment.
To cope with such a problem, Japanese Patent Application Laid-open No. 2001-235898 discloses a spherical toner using a magnetic powder containing phosphorus elements. This toner is superior in resolution and in durability in a high-temperature and high-humidity environment. However, there has been room for further improvement when used in the intermittent mode with a low print percentage in a high-temperature and high-humidity environment and a low-temperature and low-humidity environment.
In addition, from the viewpoint of miniaturization, in addition to a method of miniaturizing the main body and component parts of a developing assembly, a low-consumption toner is sought which enables a large number of sheets to be printed in a small quantity. In respect to such a low-consumption toner, it is proposed to make toner particles spherical so as to improve transfer efficiency to achieve the objective. However, in such a toner with particles made spherical, the particle surfaces have been smoothened more than those of conventional pulverization toners, and also a magnetic material may easily be enclosed inside particles. Hence, the toner tends to be unstably charged. This concurrently tends to cause faulty images such as sleeve ghosts, blotch phenomenon and density non-uniformity.
To cope with this problem, Japanese Patent Applications Laid-open No. 2003-57951 and No. 2002-311636 disclose a method in which a quaternary ammonium salt compound capable of charging iron powder positively is added to the resin coat layer of the developer carrying member to prevent a toner subjected to spherical treatment or a negative toner produced by polymerization from being charged in excess. The use of such a method can be effective in preventing the charge-up phenomenon during long-term service and in improving uniformly charging properties. However, if the quaternary ammonium salt is added in a large quantity, the strength of the resin coat layer may be lowered to tend to cause variations of surface roughness.
In general, monochrome printers or copying machines often reproduce letters or characters, where the toner consumption can be cut down by controlling what is called the line toner laid-on level (the toner amount for development with which a line image is formed). However, for example, in an attempt to form line latent images of 200 μm in width and control the toner consumption, there has been such a problem that the line width obtained actually is considerably smaller than 200 μm, resulting in a lowering of the reproducibility of latent images.
In Japanese Patent Application Laid-open No. H01-112253, it is proposed that the toner consumption can be cut down by using a toner having specific fine-powder content, true density and residual magnetization. However, such a toner tends to give a low solid-image density, and an attempt to make the image density higher results in an increase in toner consumption and also makes the lines thicker inevitably. That is, it is sought to keep the image density high and reproduce line images faithfully to latent images while cutting down the toner consumption.